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Reddy & Baugnon

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operative approach and allows the use of an intra operative image guidance system. 18 Patients should be scanned with multidetector row CT in the supine position with a field of view to include the paranasal sinuses and temporal bones. Contin uous thin-section axial images of submillimeter (ie, 0.625 mm) collimation (volumetric) should be reconstructed in the bone algorithm, and sagittal and coronal reconstructions of the raw data should be performed. 17,19 One of the greatest strengths of HRCT in the evaluation of CSF leak is that an active leak does not need to be present at the time of im aging to be able to identify an osseous defect. How ever, if the patient has multiple osseous defects, it can be challenging to determine which defect is the definite source of the CSF leak, because the presence of an osseous defect is not always asso ciated with a concomitant dural dehiscence. How ever, if only 1 osseous defect is identified and the location of the suspected leak on imaging corre sponds with the clinical symptoms, no additional imaging is needed, and the patient can proceed to surgical repair. 20 Contrast-enhanced CT cisternography (CTC) is performed by instilling intrathecal nonionic myelo graphic iodinated contrast and scanning the si nuses in the prone and supine positions, with supine images also obtained before contrast injec tion for the purposes of comparison. In a positive study, there is extracranial fluid or soft tissue den sity adjacent to an osseous defect showing 50% or greater increase in Hounsfield units on the post contrast scan compared with the precontrast scan, suggestive of interval contrast pooling. When intro duced in 1977, CTC was considered the study of choice to evaluate CSF fistulae, but it is now selec tively used as a problem-solving tool in specific scenarios, primarily in the setting of multiple osseous defects on CT, to determine the site of leak. 21,22 CTC has a wide range of reported sensi tivities of 33% to 100% and specificity of approxi mately 94%. 8,13,23–25 The main limitation of CTC is that patients have to be actively leaking, or able to elicit a leak, at the time of examination. Low rates of sensitivity are predominantly attributed to imag ing in the absence of an active leak, with other po tential causes being obscuration of small leak in the setting of high-density contrast media adjacent to high-density bone and high viscosity of contrast media prohibiting leakage through a fistulous tract. 25,26 The disadvantages of CTC include high radiation dose related to multiple scans, inherent risk of a lumbar puncture, and potential adverse outcome from iodinated contrast. Computed Tomography Cisternography

techniques used to diagnose and characterize the site of a CSF leak and then details the pathophys iology and associated imaging findings in trau matic and spontaneous leaks. In addition, it discusses some challenges in the diagnosis of initial and recurrent leaks with an emphasis on in formation that is most consequential to referring surgeons. The first diagnostic study to evaluate a patient with CSF rhinorrhea or otorrhea and suspected CSF leak is testing a sample of the fluid for b 2-trans ferrin, a protein specific to the CSF, because this is the most reliable confirmatory test for a CSF leak. 8 As discussed previously, rhinorrhea can be a sign of a defect along either the paranasal si nuses or mastoids. Frank otorrhea draining from the external auditory canal in the setting of a tegmen defect within the middle cranial fossa is rare, unless there is a perforation of the tympanic membrane (ie, in the setting of trauma), or a tym panostomy tube. Various methods of testing for b 2-transferrin report sensitivities of 87% to 100% and specificities of 71% to 94%. 9 Patients with intermittent leaks may be able to collect an adequate volume of sample themselves over the course of a week, if necessary, without storage re strictions to prevent protein degradation. 10 Although not widely used in the United States, beta trace protein is another CSF marker, and some recent studies report that it has a higher sensitivity and specificity than b 2-transferrin with lower cost and faster turnaround time. 9,11,12 Once the leak is confirmed, localization and char acterization can be achieved with radiologic evaluation. High-resolution CT (HRCT) of the paranasal si nuses and mastoids should be the first line of imag ing because computed tomography (CT) is the best modality to delineate osseous anatomy with the greatest spatial resolution to pinpoint a site of bony dehiscence. HRCT has a reported sensitivity of 88% to 95% in identifying the site of skull base defect after the presence of CSF leak is confirmed by b 2-transferrin analysis. 13–16 In a single retro spective study at our institution, CT correctly pre dicted the site of leak in 100% of the cases when 0.625-mm axial images were available and multi planar reformations could be generated. 17 In addi tion to excellent accuracy, HRCT provides unparalleled delineation of the remaining osseous sinonasal anatomy for surgeons to plan their IMAGING PROTOCOLS COMPUTED TOMOGRAPHY

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